Oh, so angry as well over here.

There is a MASSIVE change in attitudes at most schools with regards to learning difficulties. They can easily identify where children have problems and put in place learning plans to suit individual childrens needs. What is sounds like is the school is maybe small and understaffed and as such it is easier to not get the child assessed and make the effort. (IMO)

Like the others suggested, call the Dept of Ed and get some help from them but first take it to the principal. DofE likes to go through all the correct channels. There are so many things they can do to help and excluding your DS from the excursion is going to reinforce to him that he is different to other children and that is just not fair or right. If it helps, ask for a meeting, write down everything you want to say and then speak to them about it. If nothing happens at least you have info to take to the DofE to support your case.

GL